Various systems have been proposed for the recovery of methane-rich gases from landfills. Such systems are described, for example, in Johnson et al U.S. Pat. No. 4,026,355 and in the copending application, Ser. No. 355,478 entitled Landfill Gas Recovery Method, now U.S. Pat. No. 4,442,901. In both of these systems, a gas collection zone is maintained at a pressure lower than the natural (positive) pressure of the gas within the landfill and, preferably, below ambient. The collection zone may take the form of one or more wells or of a shallow grid of collection pipes; in either event, however, the pressure reduction in the collection zone is limited by the need to prevent atmospheric intrusion, i.e., the sucking of outside air into the landfill where its oxygen content poisons the methane-producing organisms. Additionally, the nitrogen content of the air introduced to the landfill reduces the heating value of the processed landfill gas.
A partial solution to this problem has been proposed in the aforesaid pending application by providing a gas barrier within the cover layer of the landfill above the collection zone. This solution is adequate if the landfill is reasonably homogeneous. In practice, however, that is seldom the case, as landfills by their very nature tend to be extremely heterogeneous and often contain large fissures through which pressure differences can propagate rapidly.
It is, of course, desirable in the operation of a landfill gas recovery system to maintain the gas collection zone at the lowest allowable pressure so as to extend the zone of influence of the collector (i.e. the volume from which it collects gas) as far out as possible. In a typical landfill, a well, group of wells, or collector grid will thus create a low-pressure zone from which low-pressure areas extend unevenly in different radial directions. The resulting distortion of the zone of influence may well result in the inadvertent extension of the zone of influence beyond the confines of the gas barrier at one or more locations. If this is the case, the ability of the system to collect gas is significantly impaired because the collection pressure must then be kept at a level high enough to prevent atmospheric intrusion and this is too high to maximize gas collection.